A new standard for digital telecommunications transmission is undergoing development. This new standard is variously known and referred to as the "synchronous optical network" or SONET; it is also sometimes referred to as the "synchronous digital hierarchy" or SDH. Definitions of this evolving standard appear in ANSI T1.105-1988, American National Standard for Telecommunications--Digital Hierarchy--Optical Interface Rates and Formats Specification; CCITT Recommendation G.708, Network Node Interface for the Synchronous Digital Hierarchy; and CCITT Recommendation G.709, Synchronous Multiplexing Structure, the disclosures of which are incorporated herein by reference. These standards currently are not wholly consistent, and they are still undergoing evolution in the respective organizations. However, the differences remaining between these standards are not pertinent to the present invention, and these standards provide those skilled in the art with a working background knowledge against which this invention will be more fully understood and appreciated.
These standards specify methods and suggest apparatus by which lower-rate digital transmission streams may be multiplexed as encapsulated data within higher-rate digital transmission streams for higher speed transmission. In North America, such lower rate streams are called "virtual tributaries" or "VTs," while the CCITT specifications refer to them as "virtual containers." For purposes of this disclosure, these terms are deemed synonymous. These lower rate digital transmission streams include, for example, 24-channel DS1 time-division-multiplex telephone services. A survey paper entitled "Synchronous Networking--SONET and the SDI" included within a colloquium entitled The Changing Face of Telecommunications Transmission, presented by Professional Group E7 (Telecommunications Networks and Systems), Institution of Electrical Engineers, London, England, Jan. 16, 1989, pp. 6/1 to 6/27, provides an overview as to how the new synchronous transmission standards may be used to provide a managed transmission infrastructure for flexible allocation of transport capacity among various service networks. The disclosure of this paper is also incorporated herein by reference.
Without loss of intended generality, the following discussion adopts and uses the North American terms and conventions. Thus, these standards are collectively referred to herein by the acronym "SONET." The subordinate information carriers are called "virtual tributaries" or "VTs." The concrete examples refer to 64 kilobit per second channels (DS0s) comprising 8 bit bytes transmitted at a frequency of 8 kHz (e.g. conventional digital voice frequency transmissions), the DS0s being encapsulated within so-called VT1.5s, while recognizing that the principles of the present invention apply identically to all forms of channels and VTs, not simply to DS0s and VT1.5s, and in fact to non-SONET transmission systems, e.g. DS1s.
Classically, a frame for time division multiplexing within telephony corresponds to a period of 125 microseconds, resulting in a frequency of 8 kHz. Consistent with this practice, a SONET frame occupies 125 microseconds. It comprises a matrix of eight-bit bytes. There are always nine rows and 90*N columns in the matrix, where the value of N is defined by the number of bytes actually included within the frame, i.e. the multiplexing level or data density (e.g. OC1, OC3, etc.). The choice of multiplexing level N and resultant data density does not directly bear upon or affect the present invention.
By convention each SONET frame contains overhead bytes and a so-called synchronous transport signal synchronous payload envelope, or "STS SPE." For example, at a lowest multiplex level, called optical carrier one (OC1), N equals one, there are three columns in the SONET matrix for transport overhead bytes, and there are 87 columns for the STS SPE payload. The transport overhead contains a pointer that indicates some byte position marking the beginning of the STS SPE frame. From this particular byte position, the STS SPE frame extends for the next 125 microseconds, skipping over the SONET matrix transport overhead bytes that also occur during this time.
Ideally, a communications network would be driven by a universal single clock frequency and phase without any local deviations. In practice the universal single clock has not yet been realized, though deviations can be specified and controlled. In comparison with the SONET time (OC-N) frame, the STS SPE is in essence another frame all by itself. The first byte of the STS SPE frame begins at a starting time, such as t=0, and the STS SPE frame repeats 125 microseconds later. With slight frequency variations between the SONET clock and the STS SPE clock, the STS SPE frame will move backwardly or forwardly relative to the SONET frame. Thus, the STS SPE starting byte position may not be aligned with the beginning of the SONET frame at a cross-connect location or node. Rather, the STS SPE may be shifted in time relative to the beginning of the SONET frame, and, as noted, such shift is taken into account and thereby controlled by the pointer contained in the SONET frame transport overhead bytes.
The pointer mechanism includes a definition of ways to adjust its value such that one fewer byte or one extra byte can be included in the STS SPE, as needs be, and thereby permit the STS SPE frame to move relative to the SONET frame to compensate for these slight frequency variations. It is an advantage of SONET over certain older multiplexing technologies that no payload data loss or duplication occurs in the presence of small frequency differences. The present invention preserves this advantage of SONET.
An STS SPE can transport payload in a variety of formats, known as mappings. Although the present invention is applicable to any transmission system in which time-floating payload fragments can be identified, the subsequent discussion concerns itself particularly with a subset of mappings in which the STS SPE carries a plurality of virtual tributaries, or VTs. A VT comprises a VT synchronous payload envelope (VT SPE), as well as so-called VT overhead. The actual payload is structured in 125-microsecond frames contained within the VT SPE.
VTs come in different sizes. For example, a so-called VT1.5 can transport the payload of a DS1 voice grade time division multiplex digital transmission system, so there can be 24 64-kbit/sec DS0 payload channels within a VT1.5; the STS SPE of an OC1 is capable of transporting 28 VT1.5s. Each VT occupies a fixed, known set of columns in the STS SPE. Given a knowledge of the VT structure (VT1.5, VT2, etc.), the bytes comprising any given VT can be extracted through comparatively straightforward logical operations, readily apparent from the specifications cited above. This logic must track STS SPE pointer adjustments, such that when the STS SPE shifts in time, the VTs remain in synchronism with it. Although the VT as a whole is comparatively easy to extract, locating specific bytes within the VT is more complex, and is the subject of the present invention.
SONET defines two major modes in which VTs may be encapsulated: "floating" and "locked." Locked mode assumes that there is no frequency or phase difference between the STS SPE clock and the VT SPE clock. In locked mode, any and all of the VT payload is totally known simply by counting time slots from the start of the STS SPE. Locked mode is optimized to provide DS0 visibility, at the expense of delay required to align the payload with the STS SPE. In addition, if there is a frequency difference between the payload clock and the STS SPE clock, locked tributaries are kept in alignment with the STS SPE by repeating or dropping an entire payload frame from time to time, thereby affecting the integrity of the payload data path. This is referred to as "slip buffering."
To avoid these disadvantages, the SONET standards allow for a third separate layer of timing, also nominally at an 8 kHz frame rate, the third layer allowing VTs to float relative to the STS SPE: the floating VT mapping permits the VT SPE to be transported with timing that differs in phase and frequency from the STS SPE clock. Floating VTs can be transported through SONET systems with minimal delay and without slip buffering. In much the same way that the SONET overhead provides a pointer that permits the STS SPE to shift relative to the SONET frame without loss of STS SPE integrity, each floating VT contains a pointer that locates its VT SPE, independent from the SPEs of other VTs, which have their own pointers. A VT SPE pointer is meaningless and hence undefined in the locked VT mapping which doesn't utilize this third layer of timing.
The VT pointer is located at the top of the columns of the STS SPE frame associated with a particular VT. The VT pointer selects a byte that defines the beginning of its VT SPE, contained within its set of columns (e.g. 3 columns for a VT1.5). Again, pointer adjustment rules are defined to permit the VT SPE frequency to differ from the STS SPE frequency, and payload integrity is not affected by small frequency differences. Also the pointer minimizes delay even if the frequency is the same, by allowing payload to be inserted into the VT SPE at an arbitrary time/phase.
Sub-modes of these two major modes include "bit-synchronous," "byte-synchronous," and "asynchronous." Only byte-synchronous traffic is amenable to DS0 processing, and is the focus of the present invention. The other sub-modes can be readily cross-connected as entire VTs without making use of the present invention, but no DS0 rearrangement or cross-connections would occur.
Floating VTs are optimized for cross-connect of entire VT SPEs as single units, inasmuch as cross-connect delay is minimized, payload integrity is assured, and DS0 visibility is not required. Floating VTs do not easily support cross-connection of individual DS0s or groups of DS0s, because a given payload byte may appear anywhere within the columns that define a particular VT, and its location may change from time to time. Conventionally, it has been accepted that to achieve DS0 cross-connection, floating VTs must first be locked to some common timing reference, which has a disadvantage of incurring a typical delay of one additional frame, and may affect payload integrity if clock frequency differences exist. Both of these effects are undesirable, especially if allowed to accumulate through a series of multiplex and cross-connect pieces of equipment.
DS0 cross-connect directly from floating VTs is an aim of the present invention. Since the invention has far greater application beyond SONET applications, more generally an aim of the present invention is to cross-connect individual bytes or arbitrary fragments of a digital stream, one SONET example being groups of DS0s sized smaller than a VT, without first aligning a frame phase of the digital stream containing the bytes or fragments with any common frame phase timing reference, such as a frame timing associated with the cross-connect element itself or a frame phase timing of any other incoming digital stream. As used herein, the term "byte" means any predetermined arbitrary number of bits operated on as a unit by processors associated with the cross-connect element. A commonly accepted size for a byte is 8 bits (e.g. DS0), though differing communication standards use differing bit numbers as operational units. Though the invention has utility beyond SONET, since a preferred embodiment is for use with SONET it will be further explained by reference to SONET systems.
FIG. 1 is a graphic representation of a SONET frame containing an STS SPE that encapsulates a plurality of floating byte-synchronous VT1.5 SPEs that encapsulate a plurality of channels (DS0s) along with associated transport and path overhead and pointers.
Digital network elements include cross-connects, add-drop or terminal multiplexes, switches, subscriber carrier terminals and any other signal processing, checking or regenerating node. The function of a digital cross-connect is to terminate standard digital multiplexed signals and to cross-connect constituents thereof under either a local or a remote control mechanism. The current state of digital cross-connect technology is summarized in a paper by B. D. Bowsher entitled "The Evolution of DCS Technology in Access and Core Networks," published in the Conference Record, IEEE International Conference on Communications '88, Philadelphia, Pa., held Jun. 12-15, 1988, Vol. 1, pp. 349-354. A technique for synchronizing and cross-connecting a low speed payload with a high speed data loop is described in European Patent Application 0 305 992. The disclosures of this Bowsher paper and patent application are incorporated herein by reference.
While other digital network elements typically require the capability to cross-connect digital signals only at VT and higher levels, both digital telephone network switches and digital loop carrier systems require cross-connect capability at the DS0 level, i.e. individual byte level. However, digital telephone network switches will provide floating byte-synchronous VT1.5s to digital loop carrier systems. See, for example, Bellcore, TR-TSY-000303, Supplement 2, Issue 1, October 1989, IDLC System Generic Requirements, Objectives and Interface: Feature set C-SONET Interface, 4.4.2. The disclosure of the entire Bellcore TR-TSY-000303 paper is incorporated herein by reference, in addition to 4.4.2.
FIG. 2 illustrates a block diagram of typical multiple inputs associated with a cross-connect element, showing the various timing interfaces that may exist. The subsequent example discussions assume cross-connection between DS0 #9 of VT #7 of transmission facility #5 and DS0 #2 of VT #4 of transmission facility #6, as illustrated in FIG. 2.
For a network element to cross-connect DS0s or fragments of a floating virtual tributary payload signal according to the prior art, a minimum of one frame delay is required to align the digital stream containing the DS0s with a local frame reference as a necessary part of the processing overhead associated with that particular element in converting from floating to locked mode. An additional frame delay then occurs during the actual cross-connect operation. As digital network elements proliferate within the worldwide communications grid, the frame delays accumulate. Cumulative frame delays over an extended communications path contribute to echo, a very undesirable drawback.
By way of further explanation, if a system were cross-connecting an entire STS SPE from a first transmission facility to a second transmission facility whose SONET frame had a beginning displaced in time from the beginning of the SONET frame of the first transmission facility, a given byte in the STS SPE of the SONET frame of the first transmission facility could be located by a particular row and column number relative to the incoming SONET frame. Relative to the outgoing SONET frame, that same byte could be defined with the same row and column number, but at the expense of an unnecessary delay of up to one additional frame. Alternatively, the byte could be defined to the second transmission facility at the same instant but with a different row and column number. By adjusting the STS SPE pointers for this offset, this byte can be copied from the input frame to the output frame without the additional delay, i.e. there will not be a frame latency. This characteristic is a part of the SONET specification, and results in delay minimization by providing for cross-connection of STS SPEs as integral units.
Similarly, floating VTs can also be cross-connected as integral units, this process essentially being one layer deeper than STS SPE cross-connection as integral units, and requiring both STS and VT pointers to be simultaneously accounted for.
As mentioned, if it is desired to cross-connect an individual channel, byte, or a VT fragment from a VT, the traditional approach has been to lock or align the VT with the digital network element's common system timing. This process introduces the additional frame of delay previously mentioned and may impair the integrity of the payload, since possible frequency drift between the digital network element's common system timing and VT SPE clocks can only be accommodated by dropping or repeating payload frames (slip buffering).
According to this prior practice, DS0 cross-connect equipment locks floating VTs relative to the digital network element's common system timing, with an associated overhead cost in terms of hardware, transmission delay, and payload quality degradation. It is clearly desirable to be able to cross-connect individual DS0s or groups of DS0s without having to lock their VTs.
Therefore, a hitherto unsolved need is for a new method to realize and achieve cross-connect for DS0s, bytes, or payload fragments (VT fragments) between floating VTs with no appreciable increase in implementation cost, compared to the cost of cross-connecting DS0s in locked VTs.